sched: make sure scheduler parameter change handler is first

This is needed to allow other handlers to add new timers.

local.c

@@ -252,6 +252,14 @@ void LCL_RemoveParameterChangeHandler(LCL_ParameterChangeHandler handler, void *
 
 /* ================================================== */
 
+int
+LCL_IsFirstParameterChangeHandler(LCL_ParameterChangeHandler handler)
+{
+  return change_list.next->handler == handler;
+}
+
+/* ================================================== */
+
 static void
 invoke_parameter_change_handlers(struct timeval *raw, struct timeval *cooked,
                                  double dfreq, double doffset,

local.h

@@ -92,6 +92,9 @@ extern void LCL_AddParameterChangeHandler(LCL_ParameterChangeHandler handler, vo
 /* Remove a handler */
 extern void LCL_RemoveParameterChangeHandler(LCL_ParameterChangeHandler, void *anything);
 
+/* Check if a handler is invoked first when dispatching */
+extern int LCL_IsFirstParameterChangeHandler(LCL_ParameterChangeHandler handler);
+
 /* Function type for handlers to be called back when an indeterminate
    offset is introduced into the local time.  This situation occurs
    when the frequency must be adjusted to effect a clock slew and

sched.c

@@ -514,6 +514,10 @@ handle_slew(struct timeval *raw,
   int i;
 
   if (change_type != LCL_ChangeAdjust) {
+    /* Make sure this handler is invoked first in order to not shift new timers
+       added from other handlers */
+    assert(LCL_IsFirstParameterChangeHandler(handle_slew));
+
     /* If a step change occurs, just shift all raw time stamps by the offset */
     
     for (ptr = timer_queue.next; ptr != &timer_queue; ptr = ptr->next) {